Known inertial reference units (IRU) may employ a Hemispherical Resonator Gyro (HRG) and use digital control loop algorithms which may operate in two distinct modes: Force to Rebalance (FTR) and Whole Angle (WA). The FTR mode provides high performance angular rate data by caging a resonant standing wave of the HRG by rebalancing it with electrical force. The WA mode, which allows the standing wave to rotate in an open loop fashion, provides extremely stable angle readout because its scaling depends only on its hemispherical geometry of the sensor for stability.
Force to Rebalance scaling is dependent on the stability of many different sources, such as forcer to resonator gaps, pickoff to resonator gaps, −100v resonator bias voltage, analog gain of pickoff readout, A/D voltage reference, pulse width modulated (PWM) forcer signal characteristics, and others. These effects degrade the FTR scale factor (SF) over time and with temperature. The scaling of the resonator flex pattern motion relative to the rotation of the gyro in inertial space, the geometric scale factor, is dependent only upon the geometry of the resonator and accordingly is very insensitive to thermal expansion effects, material property effects, etc. Previous testing has shown the geometric scale factor to be stable down to 0.02 ppm.
A closed loop scale factor (CLSF) error correction technique may be employed to measure and correct the HRG scale factor errors when using the force to rebalance mode of operation. The existing closed loop technique employs a deterministic 125 Hz rate square-wave modulation signal which is summed with the FTR loop command. In known closed loop calculations, it is assumed that the square-wave modulation signal is above the bandwidth of a rate control loop for the HRG, limiting the useable bandwidth of the gyro output. Also, the presence of inertial rate inputs that are correlated to the square-wave modulation signal may create large transients in the scale factor estimation, causing errors in the system output.
Thus, there is a need for reduced correlation of the input signal modulation with the inertial rate input.